A valve timing control device as disclosed in Patent Document 1 cited below has been known in the field. In the device of Patent Document 1, a drive plate 3 driven by the crankshaft of the engine is assembled such that the drive plate 3 is rotatable relative to a flange ring 7 coupled to the camshaft 1 of the device. Integrally mounted to the camshaft 1, to the front side of the drive plate 3, are a lever shaft 10 having three levers 9 and a hold ring 12, which are securely fixed to the flange ring 7 with a bolt 13. A middle rotor 23 is rotatably mounted on the hold ring 12 via a thrust bearing 28 to the front side of the lever shaft 10.
A link 14 is rotatably connected at one end of each of the three levers 9 with a pin 15. Formed at the other end of the link is an axial receptacle hole 16 for receiving therein movable members 17. A radial slot 8 (serving as a radial guide) is formed in the front end of the drive plate 3. Three spiral slots 24, each spiraling in the direction of rotation of the drive plate 3 with decreasing radius, are formed on the rear end of the middle rotor. The movable members 17 are provided at three positions in association with the three corresponding spiral slots 24. Each of the movable members 17 has retainers 19 and 21 for rotatably holding balls 18 and 20 in the respective radial slot 8 and spiral slot 24 via a leaf spring 22.
Provided on the front end of the middle rotor 23 is a permanent magnet block 29 having N- and S-poles that alternates along the circumference of the rotor 23. Arranged in front of the permanent magnet block 29 is a yoke block 30 having a first pole tooth ring 37 and a second pole tooth ring 38 for generating different magnetic poles when electromagnetic coils 33A and 33B are energized. The magnetic poles of the pole tooth rings 37 and 38 are switched on and off in a given switching pattern by the middle rotor 23 so as to apply changing magnetic forces on the permanent magnet block 29 to rotate the drive plate 3 relative to the camshaft 1. The rotation of the drive plate 3 is terminated by ending switching of the polarities.
As the middle rotor 23 is angularly advanced in relative to the drive plate 3 in the rotational direction R (referred to as angularly advancing direction) under the polarity switching of the polar tooth rings 37 and 38, the balls 18 and 20 of the movable member 17 are displaced radially outward in the respective radial slot 8 and spiral slot 24. Then, the lever shaft 10 is retarded in relative to the drive plate 3. That is, the level shaft 10 rotates in the angularly retarding direction (opposite to the rotational direction R of the drive plate 3), thereby rendering the rotational phase of the crankshaft and camshaft 1 retarded in the angularly retarding direction. On the other hand, when the polarity switching pattern of the polar tooth rings 37 and 38 is changed so as to delay the middle rotor 23 in the angularly retarding direction, the movable member 17 is displaced radially inward, thereby rendering the rotational phase of the crankshaft and camshaft changed in the angularly advancing direction.
During an operation, the camshaft 1 is subjected to reactions of the valve springs, which cause disturbing torques on the camshaft. Such disturbing torques may cause unexpected angular displacements of the drive plate 3 relative to the camshaft 1. The device of Patent Document 1 has a self-lock mechanism in which the camshaft 1 is immovably locked to the drive plate 3 via the link 14 and the lever 9 by pushing the ball 20 in the direction perpendicular to the spiral slot 24 against the inner wall of the spiral slot 24 when a disturbing torque occurring in the camshaft 1 is transferred to the movable member 17 via the lever 9 and the link 14, causing the ball 18 to be displaced in the radial slot 8 in the direction perpendicular to the spiral slot 24.